Nuclear fusion is the process by which two or more nuclei fuse to make a larger nucleus.[1] The successful fusion of two light (lighetr than iron) nuclei releases large amounts of energy, but to start the reactions large amounts of energy must be input to overcome the repulsive electrostatic forces.

In the center of most stars, hydrogen fuses together to form helium. Fusion in stars releases so much heat that the process alone keeps their mass from collapsing in on itself due to gravity. It is the reason that stars are so stable as well. If the core of a star starts to collapse, more fusion reactions occur and it expands again from the heat. If the core expands too far, less fusion reactions occur and it collapses back down.

Scientists here on earth are trying to make nuclear fusion in the laboratory a useful energy source. One example is the European Toroidal Reactor near Cambridge in England. If successful fusion would supply a vast amount of clean energy, as the fuel would be derived from seawater and the by-product would be helium. Uncontrolled fusion causes thermonuclear explosions, which are used in the hydrogen bomb.